In the fields of the electronic and electrical industry and the automobile, aircraft and space industries, there is a strong demand in recent years for crystalline thermoplastic resins having high heat resistance of about 300.degree. C. or higher in terms of melting point and moreover easy melt processability. Polyether ketones having predominant recurring units of the following structural formula [I] or [II]: ##STR5## were discussed [Polymer, 21, 577 (1980)].
These polyether ketones have excellent heat resistance and mechanical strength. However, they use expensive fluorine-containing monomers and utilize, as a solvent, an aromatic sulfone which is costly upon its separation and purification from the resulting polymers. The production process thereof thus involves many disadvantages in its industrial use (Japanese Patent Publication No. 22938/1982).
Besides, as poly(arylene thioether-ketone) type polymers, there have been proposed polymers having predominant recurring units of the following structural formula [III], [IV], [V] or [VI]: ##STR6##
The poly(arylene thioether-ketone)s (hereinafter abbreviated as "PTKs") having the predominant recurring units of the structural formula [III] have excellent heat resistance, but involve a problem that they have poor stability upon melting (Japanese Patent Laid-Open Nos. 58435/1985 and 124/1989).
The polymers having the predominant recurring units of the structural formulae [IV] and [V], respectively, are not suitable for industrial production because they must use particular polymerization solvents and monomers (Japanese Patent Laid-Open Nos. 200127/1986, 197634/1986 and 27434/1987).
The poly(arylene thioether-ketone-ketone)s (hereinafter abbreviated as "PTKKs") having the predominant recurring units of the structural formula [VI] have a melting point as extremely high as about 410.degree. C. This is however not all good. Their melt processing temperature are high accordingly, so that they tend to loss their crystallinity or to undergo crosslinking and/or carbonization, resulting in a rapid increase in melt viscosity, upon their melt processing.
In addition, since PTKKs contain ketone groups in their recurring units, they are poor in solvent resistance and moisture absorption resistance, so that their application fields as heat-resistant resins are unavoidably limited. PTKKs are generally obtained as fine powders. This has led to an additional problem upon their production such that they show poor handling properties in their collection step after polymerization, especially in filtration, washing, drying and transportation. Still further problems have also arisen such as poor metering property upon melt processing and occurrence of blocking in hoppers or the like.
On the other hand, for example, poly(p-phenylene thioether) as a poly(arylene thioether) (hereinafter abbreviated as "PATE") is used as high-performance engineering plastics having excellent heat resistance and solvent resistance. This polymer is obtained by reacting dichlorobenzene, which is very cheap and industrially available, with sodium sulfide (U.S. Patent Specification No. 3,919,177). However, its crystalline melting point is about 285.degree. C. and its glass transition point (Tg) is also as low as about 85.degree. C. There is thus a demand for development of polymers having a higher melting point and a higher Tg.
In order to solve the above problem, there has also been proposed copolymers in which arylene thioether units and sulfone units of the formula ##STR7## or ketone units of the formula ##STR8## are introduced at random therein (Japanese Patent Publication No. 5100/1984).
It is however impossible to obtain polymers having satisfactory uniformity in composition, heat resistance and/or melt stability by the prior art process in which a dihalobenzene and a dihalogenated aromatic compound activated by the ketone group or sulfone group are reacted together with an alkali metal sulfide in a polar organic solvent to copolymerize them, because their reactivity and chemical stability in a polymerization system are different from each other. Namely, the resultant random copolymers tend to have lower crystallinity and poorer heat resistance and mechanical properties as the proportion of the arylene thioether units decreases, in particular, to 90 mole % or less.
It has been proposed to react an aromatic thioether with phosgene or an aromatic dicarboxylic acid dihalide in the presence of a Lewis acid in an aprotic solvent, thereby obtaining polymers having respective predominant recurring units of the following structural formulae [VII] and [VIII] (Japanese Patent Laid-Open Nos. 104126/1985 and 120720/1985): ##STR9## However, the resulting polymers were accompanied by problems that they have a low degree of polymerization and poor melt stability, and undergo gelation at once.